Egbert Giles Leigh

The average lifetime of a population in a varying environment

Abstract I derive an approximate formula relating the average time to extinction of a population in a varying environment to its initial size, its equilibrium size (if it is self-regulated), its innate capacity for natural increase, and the impact upon it of environmental variation. The greater the impact of environmental variation, the more slowly a populations prospective lifetime increases with increase in its equilibrium size. The lifetimes of populations greatly influenced by environmental variation are more sensitive to the relative amplitude of fluctuation in their numbers than to their equilibrium size. Since species tend to avoid competitive displacement by specializing, rendering themselves more sensitive to environmental change, and since populations are no more likely to risk extinction in one environment than in another, the degree to which a communitys populations fluctuate will be unrelated to environmental stability.

Sex change and sexual selection.

Many writers have discussed sequential hermaphroditism, wherein individuals are all born of one sex and change to the other as they grow older, in terms of its advantage to the species (1). Ghiselin (2) shifted the focus by suggesting that if members of one sex increase in fertility more rapidly with age than those of the other, then natural selection will favor a genotype whose individuals are all born into the sex that suffers less from being young (the sex whose fertility increases

Why Do Some Tropical Forests Have So Many Species of Trees

Understanding why there are so many kinds of tropical trees requires learning, not only how tree species coexist, but what factors drive tree speciation and what governs a tree clade’s diversification rate. Many report that hybrid sterility evolves very slowly between separated tree populations. If so, tree species rarely originate by splitting of large populations. Instead, they begin with few trees. The few studies available suggest that reproductive isolation between plant populations usually results from selection driven by lowered fitness of hybrids: speciation is usually a response to a “niche opportunity.” Using Hubbell’s neutral theory of forest dynamics as a null hypothesis, we show that if new tree species begin as small populations, species that are now common must have spread more quickly than chance allows. Therefore, most tree species have some setting in which they can increase when rare. Trees face trade-offs in suitability for different microhabitats, different-sized clearings, different soils and climates, and resistance to different pests. These trade-offs underlie the mechanisms maintaining a-diversity and species turnover. Disturbance and microhabitat specialization appear insufficient to maintain a-diversity of tropical trees, although they may maintain tree diversity north of Mexico or in northern Europe. Many studies show that where trees grow readily, tree diversity is higher and temperature and rainfall are less seasonal. The few data available suggest that pest pressure is higher, maintaining higher tree diversity, where winter is absent. Tree a-diversity is also higher in regions with more tree species, which tend to be larger, free for a longer rime from major shifts of climate, or in the tropics, where there are more opportunities for local coexistence.

Formation of the Isthmus of Panama

Independent evidence from rocks, fossils, and genes converge on a cohesive narrative of isthmus formation in the Pliocene. The formation of the Isthmus of Panama stands as one of the greatest natural events of the Cenozoic, driving profound biotic transformations on land and in the oceans. Some recent studies suggest that the Isthmus formed many millions of years earlier than the widely recognized age of approximately 3 million years ago (Ma), a result that if true would revolutionize our understanding of environmental, ecological, and evolutionary change across the Americas. To bring clarity to the question of when the Isthmus of Panama formed, we provide an exhaustive review and reanalysis of geological, paleontological, and molecular records. These independent lines of evidence converge upon a cohesive narrative of gradually emerging land and constricting seaways, with formation of the Isthmus of Panama sensu stricto around 2.8 Ma. The evidence used to support an older isthmus is inconclusive, and we caution against the uncritical acceptance of an isthmus before the Pliocene.

Sex Ratio and Differential Mortality between the Sexes

Differential mortality between the sexes after the period of parental care apparently does not alter the ratio of male to female births. A continuous model which keeps track of the numbers of each sex shows that an allele for equal division of effort between the sexes is favored over any allele causing a slight deviation from equality, no matter what the difference in mortality between the sexes.

Neutral theory: a historical perspective

To resolve a panselectionist paradox, the population geneticist Kimura invented a neutral theory, where each gene is equally likely to enter the next generation whatever its allelic type. To learn what could be explained without invoking Darwinian adaptive divergence, Hubbell devised a similar neutral theory for forest ecology, assuming each tree is equally likely to reproduce whatever its species. In both theories, some predictions worked; neither theory proved universally true.

The evolution of mutualism.

Like altruism, mutualism, cooperation between species, evolves only by enhancing all participants’ inclusive fitness. Mutualism evolves most readily between members of different kingdoms, which pool complementary abilities for mutual benefit: some of these mutualisms represent major evolutionary innovations. Mutualism cannot persist if cheating annihilates its benefits. In long‐term mutualisms, symbioses, at least one party associates with the other nearly all its life. Usually, a larger host harbours smaller symbionts. Cheating is restrained by vertical transmission, as in Buchnera; partner fidelity, as among bull‐thorn acacias and protective ants; test‐based choice of symbionts, as bobtail squid choose bioluminescent bacteria; or sanctioning nonperforming symbionts, as legumes punish nonperforming nitrogen‐fixing bacteria. Mutualisms involving brief exchanges, as among plants and seed‐dispersers, however, persist despite abundant cheating. Both symbioses and brief‐exchange mutualisms have transformed whole ecosystems. These mutualisms may be steps towards ecosystems which, like Adam Smith’s ideal economy, serve their members’ common good.

Historical biogeography of the Isthmus of Panama

About 3 million years ago (Ma), the Isthmus of Panama joined the Americas, forming a land bridge over which inhabitants of each America invaded the other—the Great American Biotic Interchange. These invasions transformed land ecosystems in South and Middle America. Humans invading from Asia over 12000 years ago killed most mammals over 44 kg, again transforming tropical American ecosystems. As a sea barrier, the isthmus induced divergent environmental change off its two coasts—creating contrasting ecosystems through differential extinction and diversification.

Predation by Rodents and Bruchid Beetles on Seeds of Scheelea Palms on Barro Colorado Island, Panama

Predation by rodents on seeds of Scheelea zonensis was studied on Barro Colorado Island, Panama, during the latter half of this palms fruiting season. The number of Scheelea bearing fruit declined steadily during this period. The proportion of both intact and bruchid-infested seeds among those accumulated under fruiting Scheelea declined, while the proportion of gnawed seeds increased. By early October, 87 percent of the Scheelea seeds sampled had been gnawed by rodents, and few intact or bruchid-infested seeds remained below parent palms. From August through October, agoutis, Dasyprocta punctata, visited Scheelea less and less frequently, while visits by squirrels, Sciurus granatensis, did not dedine. To learn the fate of late-falling Scheelea seeds, we placed small piles of thread-marked seeds, each similar to a small Scheelea fruit crop, in the forest during three successive months, a total of 400 seed each month. Of these 1200 thread-marked seeds, 57.2 percent were retrieved within 20 m of their piles 7 days after placement. Of the retrieved seeds, 22.2 percent were gnawed within 1 m of their pile, 70.6 percent were carried between 1 and 20 m before gnawing, and 7.2 percent were cached (not gnawed, but buried in the ground or covered with litter). More seeds were gnawed, and fewer cached, in September and October than in August. On Barro Colorado Island, intense seed predation by rodents on Scheelea correlates with a previously documented forestwide low in availability of fruits of all kinds, and with a decline in egg laying by bruchids.

Can Neutral Theory Predict the Responses of Amazonian Tree Communities to Forest Fragmentation

We use Hubbell’s neutral theory to predict the impact of habitat fragmentation on Amazonian tree communities. For forest fragments isolated for about two decades, we generate neutral predictions for local species extinction, changes in species composition within fragments, and increases in the probability that any two trees within a fragment are conspecific. We tested these predictions using fragment and intact forest data from the Biological Dynamics of Forest Fragments Project in central Amazonia. To simulate complete demographic isolation, we excluded immigrants—species absent from a fragment or intact forest plot in the initial census but present in its last census—from our tests. The neutral theory correctly predicted the rate of species extinction from different plots as a function of the diversity and mortality rate of trees in each plot. However, the rate of change in species composition was much faster than predicted in fragments, indicating that different tree species respond differently to environmental changes. This violates the key assumption of neutral theory. When immigrants were included in our calculations, they increased the disparity between predicted and observed changes in fragments. Overall, neutral theory accurately predicted the pace of local extinctions in fragments but consistently underestimated changes in species composition.